Present day chillers require significant power to produce process and cooling water. Air-cooled condensing chillers draw significantly more power (higher kW/ton) than water-cooled condensing chillers (lower kW/ton). Water-cooled chillers are more efficient from a power consumption perspective; however, this efficiency comes with a high cost of water consumption by the water-cooled chillers.
Modern water-cooled chillers can be paired with a free cooling apparatus such as an apparatus including plate frame heat exchangers. The free cooling apparatus is intended to provide some supplemental cooling with or without compressor assistance during cooler seasonal operation in an effort to reduce overall power use annually. These systems require complicated water flow control systems and swing over valves to facilitate transition in and out of free cooling modes. The transition in and out of free cooling can be risky, and relies on an operator interface to mitigate risk. Typical problems arise on transition in which a chiller can lock itself out from commencing operation on low back pressure safety control when commanded to energize with low temperature condenser water present.
Present day air-cooled chillers can be provided with free cooling coils. Most of these chillers are designed with either an evaporator or free cooling coil path of water flow. As such, the transition from free cooling to mechanical cooling can be a risky transition, and require time delays and complicated control sequences.
Water chillers are produced in a highly commoditized market. Therefore, costs are a consideration and free cooling coils are rarely oversized to facilitate increased free cooling capability.
Traditional chillers rely on supporting pumping systems. These pumping systems draw significant power. They draw the second highest amount of power in a cooling system behind chiller compressors. A typical evaporator pump delivers approximately 7.6-9.1 lpm/ton (liters per minute/ton) flow to a chiller evaporator, based on a 5.6-6.7 delta T ° C. A typical condenser water pump delivers approximately 9.5-11.4 lpm/ton of flow to a chiller condenser for operation. The average water-cooled chiller system requires approximately 18.6-20.4 lpm/ton of pumping.
The operating efficiency of a traditional chiller compression cycle is subject to the external environmental conditions. The higher the ambient dry bulb temperature (air cooled) or wet bulb temperature (water cooled), the higher the compression ratio is, which elevates the “lift” that the compressor must overcome. This lift effect requires increased compressor power to overcome the lift under high-temperature operation.
The stable operation of the traditional chiller compression cycle is subject to the external environmental conditions. The lower ambient dry bulb temperature (air cooled) or wet bulb temperature (water cooled), the lower the compression ratio is, which can pose operational risks to the system. Low temperature environmental operation of a compressor can affect the stability of the refrigerant process. A system that has too close a lift condition (high pressure versus low pressure delta p) can cause instability of the refrigerant circuit and lead to systemic reliability problems. Hot gas bypass can alleviate these problems; however, this is energy inefficient.
Present day chiller plants that serve noncritical systems generally are piped with traditional single pipe systems. Critical systems that serve data centers are usually piped with dual pathways of piping, and the ability to concurrently isolate for the purposes of maintenance or replacement of any segment of the pipe circuit, or the equipment, such as the chillers. This is a costly way to build, maintain, and replace segments of the pipe circuit, and poses additional risk to the critical load when maintenance and replacement work is performed. Present day air-cooled chillers also have integrated or packaged evaporator and condensing sections.